Soil acidification is a natural process that is accelerated by agriculture. It is primarily caused through the leaching of nitrates from nitrogen fertiliser or organic matter and removal of cations in products.
The main effect of low pH in the surface (10 cm) soil is on nitrogen fixation by legume-rhizobia symbiosis and on the availability of nutrients.
In the subsurface (10-30 cm) layers low pH causes an increase in the solubility of aluminum, which is toxic to plant roots, resulting in restricted root growth and poor access to moisture and nutrients.
Soil acidity is estimated to cost broadacre agriculture $498 million per year in WA (Herbert 2009) or about 9% of the average annual crop. It is one of the few soil constraints (particularly subsurface constraints) that can be treated with appropriate management.
Susceptibility is a measure of the time before the soil pH, below the normal depth of cultivation (i.e. 10-20 cm) will drop to a level at which production will be adversely affected (Moore et al. 1998a). The critical pH level will depend on both soil type and crop, but is usually in the range 4.0-6.0.
A soil at high risk of subsurface acidification is likely to acidify below critical pH values within 15 years. A soil at low risk is unlikely to acidify below critical levels within the next 30 years.
Susceptible soils and occurrence
Naturally alkaline soils, such as those containing lime, are buffered against acidification from plants and fertilisers. Soils that were inherently acid before clearing (e.g. Acid yellow sandy earth, Non-saline wet soil, Acid shallow duplex) and in high rainfall areas have high risk of acidification from agriculture. Sandy soils which have low pH buffering capacity can acidify rapidly. Distinction can be made between pale or bleached sands which can acidify rapidly, but contain little aluminium. They would still have nutrient deficiency problems from applied fertilisers (as they are naturally infertile).
The most susceptible soil groups are Acid shallow duplex, Acid yellow sandy earth, Wet soil and Red and yellow sandy earth.
The present strategy is to add sufficient lime to the topsoil to raise the pH to >5.5. This ensures alkalinity will move down the soil profile to ameliorate acidity in the subsurface (Bolland et al. 2004).
Moore (1998a) suggests four additional strategies where acidity is or will become a problem:
- reduction of acidification rates by measures such as feeding hay back onto the paddock, limiting the rate of nitrogen leaching, minimising the use of elemental sulphur as fertiliser and adopting a less damaging crop regime;
- planting more acid-tolerant crops;
- do nothing and accept decreasing yields;
- or add extra nutrients such as N, P and K.
Key Messages across south-west Western Australia (DAFWA Report Card 2013)
Soil acidity, as indicated by the proportion of samples collected that were below the recommended pH targets, is widespread and extreme in many areas of the south-west of WA, particularly in sandy soils. Confidence in the results in most agricultural areas of the south-west of WA is considered to be high.
Condition and trend
- Soil acidity in the south-west of WA is a major constraint to agricultural production.
- Topsoil samples collected since 2005 were rated poor – the pH of more than 70% of surface soils (0-10 cm) was more acidic than recommended.
- Almost half of the samples collected from the subsurface layers (10-20 and 20-30 cm) had a pH which was rated poor – more acidic than recommended.
- The majority of agricultural soils are continuing to acidify because the annual use of agricultural lime is 40% of the estimated amount required to treat existing acidity and on-going agricultural soil acidification.
- Initial assessment of trends over the last decade indicate that the situation is, at best, static or getting worst, except for some areas with ready access to lime sources or where extension efforts have been sustained.
- The opportunity cost of lost agricultural production in the south-west of WA from soil acidity is estimated at $498 million annually. In addition, there are other off-site environmental costs.
- If soil pH drops below critical levels, root growth is restricted and crop and pasture production is severely limited. In most areas, the surface pH is below target and a significant proportion below critical. The situation is similar for the subsurface 10-20 and 20-30 cm, except in isolated areas with naturally alkaline subsoils.
- Existing soil acidity and on-going soil acidification can be economically treated with agricultural lime.
- The use of agricultural lime to manage soil acidity needs to increase and be maintained to ensure good condition and productivity of the soil resource.
Projects addressing Soil Acidification
SWCC supports a number of landholders and partners to undertake projects addressing agricultural practices including soil acidification. One project addressing soil acidification is:
- Balancing Investment in phosphorus and lime on high phosphorus and low pH soils; Southern DiRT (2014-2016).
Further information on this project can be found on the Agtrialsites website here.
The Agtrialsites website www.agtrialsites.com is a cross regional NRM WA initiative that aims to provide information on sustainable agriculture projects across Western Australia.
- Gazey C, Andrew J and Griffen E (2013). ‘Soil Acidity’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.
- Van Gool, D., Vernon, L. and Runge, W. (2008). Land Resources in the South-West Agricultural Region. A shire-based summary of land degradation and land capability. Department of Agriculture and Food Western Australia.